Thermal treatment of biomass

ABSTRACT

A biomass pyrolysis process is provided in which biomass feedstock is mixed with a heat carrier. The heat carrier at least partly comprises char. The ratio by weight of biomass to char is in the range 1:1 to 1:20. The process may be carried out by in a screw/auger pyrolysis reactor in which the solid feedstock components are conveyed along the reactor by a first screw. A second screw conveys at least a portion of the solid products of the biomass pyrolysis back to a heat transfer medium input port. Thus, the heat transfer medium includes char from the biomass pyrolysis.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/992,654, filed on Nov. 15, 2010, which is a U.S. nationalphase entry of PCT/GB09/001225, with an international filing date of May14, 2009, which claims the benefit of British Patent Application No.0808739.7, filed on May 14, 2008, the entire disclosures of which areincorporated herein by reference.

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to the thermal treatment of biomass. Ithas particular, but not exclusive, application to biomass pyrolysisreformers for the production of renewable gaseous and solid fuels frombiomass.

Related art

Biomass pyrolysis is the thermal decomposition of biomass (e.g. plantmaterial such as wood and wood bark) substantially in the absence ofoxygen. Biomass is typically a mixture of hemicellulose, cellulose,lignin and small amounts of other organics. These components typicallypyrolyse or degrade at different rates and by different mechanisms andpathways.

One traditional example of biomass pyrolysis is the production ofcharcoal, where the main product of the pyrolysis is char. Alternativebiomass pyrolysis techniques provide a product which, after cooling,includes a substantial proportion of liquid. This liquid is typically adark brown liquid having a heating value that is around one half theheating value of conventional fuel oil. The liquid is typically referredto as bio-oil. In some circumstances, it is the bio-oil which is themost valuable product of the pyrolysis reaction, since bio-oil can beeasily stored for later use, e.g. for heat and/or electricitygeneration. However, in other circumstances, the gas products may bemore useful, e.g. in rural locations for combined heat and power (CHP)applications where the gas may be used to produce electricity.

The rate and extent of decomposition of the components of biomassdepends on the process parameters of the pyrolysis reactor. In turn,these process parameters may also have an effect on the subsequentbehaviour of the product, e.g. by secondary reactions such as cracking(of higher molecular mass products) or condensation reactions (of lowermolecular mass products).

In order to produce a high proportion of gas phase by a pyrolysisprocess, it is typical to carry out a gasification-type pyrolysisprocess. In such a gasification process, it is typical to heat the solidbiomass to 300-600° C. to achieve pyrolysis of the biomass, the productsof which are solid char, condensable organic compounds (including tar),water and gases. Subsequently in the process, reactions are promotedabove about 700° C. (typically at around 800° C.) to decrease the liquid(vapour) concentration to produce further useful gas phase products, andalso to gasify some of the char via gas-solid and gas-gas interactions.There are several different types of gasifier reactor types that havebeen characterised. See, for example, A. V. Bridgwater (“Renewable fuelsand chemicals by thermal processing of biomass” Chemical EngineeringJournal Volume 91, Issues 2-3, 15 Mar. 2003, pages 87-102), whichdisclosure briefly reviews the characteristics of the following gasifierreactor types: downdraft fixed bed, updraft fixed bed, bubbling fluidbed, circulating fluid bed, entrained flow, twin fluid bed, screw/augerkiln, rotary kiln, cyclonic and vortex.

In each of the gasifier reactor types indicated above, it is necessaryto provide a heat transfer medium in order to achieve rapid andefficient heating of the biomass feedstock and the products of pyrolysisto promote gasification. Typically particles such as sand are used asthe heat transfer medium.

WO 02/50484 discloses an apparatus for the thermal treatment ofmaterial. It is primarily intended for the recycling of electronicswaste material, but can also be used for the thermal treatment ofbiomass. WO 02/50484 discloses a screw kiln in which thermallyconductive particles are provided in the feedstock. These particles arespheres of metal, ceramics or SiC. Primarily they have the function ofcleaning the interior surface of the screw kiln. On exiting the kiln,the thermally conductive particles may be re-used by being conductedback to the entrance to the kiln along a hollow shaft of the kiln.

SUMMARY OF THE INVENTION

The present inventors have realised that particular advantages may begained by using a high proportion of char in the pyrolysis process.

Accordingly, in a first preferred aspect, the present invention providesa biomass pyrolysis process in which biomass feedstock is mixed with aheat carrier, the heat carrier at least partly comprising char, theratio by weight of biomass to char being 1:1 to 1:20.

The present inventors consider that the use of high proportions of charto biomass promotes the formation of synthesis gas (syngas—a mixture ofCO and H₂) and the formation of lower organics. In general, the use ofhigh proportions of char are considered to reduce the vapour (liquid)phase proportion of the pyrolysis products. Surprisingly, this isadvantageous in some circumstances, such as in combined heat and power(CHP) applications.

Preferred or optional features of the first aspect of the invention willnow be set out. These may be combined either singly or in anycombination, unless the context demands otherwise.

Preferably the process is a continuous (or quasi-continuous) one and theratio by weight of biomass to char is the steady state ratio of thesecomponents during the process.

The upper limit for the ratio by weight of biomass to char may morepreferably be 1:1.5 or less or, more preferably still, 1:2 or less.

The lower limit of the ratio by weight of biomass to char may morepreferably be 1:15 or more or, more preferably still, 1:10 or more or1:5 or more.

Preferably, the process uses a pyrolysis reactor in which the solidfeedstock components are conveyed along the reactor. For example, thepyrolysis reactor may be a screw/auger kiln.

Alternatively, the process may use a pyrolysis reactor in which a bed ofbiomass and heat carrier is mixed but not conveyed during the pyrolysisreaction.

Preferably the process is operated at a temperature of 600° C. or lower.This is significantly lower than typical gasification temperatures(typically around 800° C.) and yet the gas phase products of the processare preferably comparable to the gas phase products of gasification.

In order to provide a high proportion of char relative to biomass in theprocess, it is preferred to recycle at least some of the char that isproduced in the pyrolysis process itself. One benefit of this is thatthe char can remain hot from its formation by the biomass process andthus can make a significant contribution to the transfer of heat to thenew biomass for pyrolysis. Thus, the char can form part (or indeed all,in some circumstances) of the heat carrier for the pyrolysis process.Alternatively it is possible to store char for later use in the reactor.The disadvantage of this is that the requirement to heat the char willreduce the efficiency of the overall process.

Similar comments apply to ash formed in the pyrolysis process—the heatcarrier in subsequent pyrolysis processes may comprise ash, at least inpart. However, it is considered that ash does not play so significant arole in the pyrolysis process as char, since it is considered that apart of the char is consumed in the reformation reaction:C+H₂O→CO+H₂for the production of synthesis gas, or syngas. Syngas has a lowerheating value than methane (natural gas) for example, but still providesa useful and convenient fuel for the subsequent generation ofelectricity in combined heat and power (CHP) apparatus.

The present inventors have realised that this use of one or more solidproducts of the biomass pyrolysis process as all or part of a heattransfer medium constitutes a separate, independent aspect of theinvention, which may be combined with the first aspect, and any otheraspects, of the invention.

Thus, in a second aspect, the present invention provides a biomasspyrolysis process including the steps:

conveying biomass feedstock and heat transfer medium from respectivebiomass feedstock and heat transfer medium input ports through apyrolysis zone of a biomass pyrolysis apparatus to produce at leastsolid and gaseous biomass pyrolysis products; and

conveying at least a portion of the solid products from an output portof the apparatus back to the heat transfer medium input port.

In a third aspect, the present invention provides a biomass pyrolysisapparatus having:

a biomass feedstock input port;

a heat transfer medium input port;

a first conveying means for conveying the biomass feedstock and heattransfer medium through a pyrolysis zone of the apparatus;

an output port for solid products of the biomass pyrolysis; and

a second conveying means for conveying at least a portion of the solidproducts of the biomass pyrolysis back to the heat transfer medium inputport, so that the heat transfer medium includes said solid products ofthe biomass pyrolysis.

Preferably, the first and second conveying means are located withrespect to each other so that one of them surrounds at least a part ofthe other. In this way, the heat of the second conveying means may serveat least partially to heat or insulate the first conveying means. Forexample, the second conveying means may be disposed annularly around thefirst conveying means. Alternatively, the first and second conveyingmeans may be located adjacent one another.

It is specifically envisaged that the second or third aspects may becombined with the first aspect, including with any combination ofpreferred or optional features of the first aspect.

Still further preferred or optional features are set out below, thesebeing combinable in any combination with any aspect of the invention,unless the context demands otherwise.

It is preferred that not all of the char produced in the biomasspyrolysis process is recycled into the biomass pyrolysis process.Preferably 90% or less (by weight), more preferably 80% or less, 70% orless, 60% or less or 50% or less is recycled into the biomass pyrolysisprocess. At least a portion of the remaining char may be combusted, inorder to provide heat for the biomass pyrolysis process. Preferably, atleast 10% of the char (by weight) is combusted in this way, morepreferably at least 20%, at least 30%, at least 40% or at least 50%.

At least a portion of the products of the pyrolysis process may beconveyed to a gasifier apparatus. Preferably the gasifier apparatusoperates at a temperature of at least 700° C. (typically about 800° C.).Preferably the gas and/or vapour products of the pyrolysis process areconveyed to the gasifier. Optionally, further biomass is introduced intothe gasifier. The gasifier is preferably a fluidised bed gasifier.Alternatively, a downdraft gasifier may be used. Preferably the biomassintroduced into the gasifier is low ash biomass, such as wood (butpreferably not wood bark). Low ash biomass is preferred, in order toavoid corrosion and/or blockage of the fluidised bed.

For example, an outlet of the pyrolysis apparatus may be connected to aninlet of the gasifier apparatus. In this way, the vapour products of thepyrolysis apparatus may be provided at the pyrolysis zone of thegasifier apparatus. The vapour products of the pyrolysis apparatus maybe substantially ash-free. This may provide efficiency benefits to thegasification process.

The pyrolysis apparatus may operate at an internal pressure aboveatmospheric pressure. For example, the pyrolysis apparatus may operateat at least 3 mbar over atmospheric pressure (typically at least 50, or200-300 mbar over atmospheric pressure, or in some cases up to about 30bar over atmospheric pressure). This allows the vapour pyrolysisproducts effectively to be pumped into the gasifier apparatus, driven bythe over pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a gas chromatography spectrum for the water-based phaseproduct of a rapeseed biomass pyrolysis process, in which the abscissais in units of time (minutes) and the ordinate is in arbitrary units ofconcentration.

FIG. 2 shows a gas chromatography spectrum for the oily phase product ofthe same rapeseed biomass pyrolysis process as analysed in FIG. 1, theaxes having a similar format to FIG. 1.

FIG. 3 show a schematic longitudinal cross sectional view of a pyrolysisapparatus according to a preferred embodiment of the invention.

FIG. 4 shows a longitudinal schematic view of a further preferredembodiment of a pyrolysis apparatus according to the present invention.

FIGS. 5-7 show longitudinal schematic views of the main components ofthe apparatus shown in FIG. 4.

FIG. 8 shows a schematic view of a pyrolysis apparatus integrated with agasifier apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER PREFERREDAND/OR OPTIONAL FEATURES

It is known that biomass pyrolysis products are complex mixtures ofdifferent compounds. It is typical for the liquid (vapour) phases ofbiomass pyrolysis to include an oily phase with relatively low watercontent and a water-based phase with relatively high water content.

FIG. 1 shows a gas chromatography spectrum for the water-based phase ofa biomass pyrolysis product in which the biomass was rapeseed, pyrolysedby an intermediate pyrolysis process. The water content of thewater-based phase was determined to be 80.42%. The results of analysisof the spectrum are set out in the following Tables 1 and 2:

TABLE 1 Summary of water-based phase wet oil dry oil Substance group wt.% wt. % Acids 2.031 10.372 Nonaromatic Alcohols 0.000 0.000 NonaromaticAldehydes 0.000 0.000 Nonaromatic Ketones 0.100 0.511 Furans 0.086 0.438Pyrans 0.000 0.000 Sugars 0.382 1.949 Benzene 0.000 0.000 Catechols0.020 0.103 Aromatic Aldehydes 0.000 0.000 Aromatic Ketones 0.000 0.000Lignin derived Phenols 0.012 0.061 Guaiacols/Methoxy phenols 0.000 0.000Syringols/Dimethoxy phenols 0.028 0.144 Miscellaneous 0.000 0.000 Total2.659 13.579

TABLE 2 Detailed analysis of water-based phase Wet oil Dry oil Compoundwt. % wt. % Acids 2.031 10.372 Acetic acid 1.598 8.161 Propanoic acid0.355 1.815 Propanoic acid, 2-methyl- * 0.015 0.076 poss: Acid compound:base mass 60, MW ? ** 0.021 0.105 Pentanoic acid, 3-methyl- ** 0.0060.030 Pentanoic acid, 4-methyl- ** 0.016 0.079 poss: Hexenoic acid **0.005 0.027 unknown aliphatic acid, MW = ? ** 0.005 0.028 poss:Heptenoic acid ** 0.010 0.052 Nonaromatic Alcohols 0.000 0.000Nonaromatic Aldehydes 0.000 0.000 Nonaromatic Ketones 0.100 0.511Butanone, 2- 0.010 0.049 3-Pentene-2-one, 4-methyl- * 0.041 0.207Diacetone alcohol (Inpurity from Acetone) * 0.017 0.0882-Cyclopenten-1-one, 2-hydroxy-3-methyl-; MCP; Cycloten 0.033 0.166Furans 0.086 0.438 Furfuryl alcohol, 2- 0.056 0.287 Butyrolactone, γ-0.022 0.112 lacton derivative 0.008 0.040 Pyrans 0.000 0.000 Sugars0.382 1.949 α-D-Glucopyranose, 1,4: 3,6-dianhydro- 0.101 0.515Arabinofuranose, 1,5-anhydro- 0.103 0.524 β-D-Xylofuranose, 1,5-anhydro-0.027 0.138 unknown Anhydrosugar ** 0.009 0.045 poss: Anhydro-d-mannosan** 0.006 0.032 unknown Dianhydrosugar ** 0.014 0.069 unknownAnhydrosugar ** 0.005 0.024 Levoglucosan; β-D-Glucopyranose, α-anhydro-0.118 0.600 Benzene 0.000 0.000 Catechols 0.020 0.103 Hydroquinone;Benzene, 1,4-dihydroxy- 0.020 0.103 Aromatic Aldehydes 0.000 0.000Aromatic Ketones 0.000 0.000 Lignin derived Phenols 0.012 0.061 Phenol0.012 0.061 Guaiacols/Methoxy phenols 0.000 0.000 Syringols/Dimethoxyphenols 0.028 0.144 Syringol; Phenol, 2,6-dimethoxy- 0.011 0.056Syringol, 4-ethyl- 0.014 0.069 Syringol, 4-vinyl- 0.004 0.019Miscellaneous 0.000 0.000 Nitrogen containing compounds 0.919 4.696Pyridine * 0.008 0.043 Pyrrole * 0.011 0.055 poss: Pyrazine, methyl- **0.008 0.041 2-Pentanone, 4-amino-4-methyl- MW 100 * 0.056 0.287 similarto Pentanone, amino-methyl- (43, 58, 100) * 0.106 0.543 Acetamide *0.051 0.260 Acetamide, N,N-dimethyl- ** 0.005 0.027 poss: Acetamide,N-methyl- ** 0.005 0.027 Propanamide * 0.013 0.067 poss: Butanamide,3-methyl- ** 0.016 0.081 Piperidone, tetramethyl- MW 155 *** 0.009 0.0472-Pyrrolidinone * 0.067 0.341 2,5-Pyrrolidinone, 1-methyl- ** 0.0130.066 poss: Pyridinone, dihydro-methyl- MW 111 ** 0.006 0.031 poss:Pentanamide, 4-methyl- ** 0.004 0.022 poss: Pyrrolidine, 1-acetyl- ***0.005 0.024 poss: Pyrazole, 5-amino-3-methyl- **** 0.004 0.021 poss:Pyridinol, methyl- ** 0.009 0.044 poss: Pyridinol or homologous ***0.107 0.545 poss: Pyrrolidinedione *** 0.055 0.283 poss: Pyridinol,methyl- * 0.007 0.033 unknown Triazine- or similar compound MW 113 ****0.012 0.059 poss: Pyridin-carboxamide **** 0.005 0.028 unknownPyrrolinone compound **** 0.005 0.025 unknown Hydantoin compound ****0.003 0.014 poss: 2,4-Imidazolidinedione, 5-methyl- MW 114 *** 0.0620.315 unknown Hydantoin compound **** 0.018 0.091 unknown Amidecompound: base mass 72, 114 MW ? **** 0.005 0.026 unknown Hydantoincompound **** 0.036 0.182 poss: Hydantoin or imidazolidinedionecompound: base mass 100 MW ? **** 0.019 0.099 unknown Piperazinedionecompound **** 0.004 0.022 unknown Piperazinedione compound **** 0.0030.016 unknown Pyrrolidin compound **** 0.015 0.076 unknown Pyrrolidincompound **** 0.012 0.059 poss: Pyrrol compound MW 154 **** 0.039 0.199poss: Pyroglutamic acid **** 0.013 0.067 unknown Hydantoin compound ****0.012 0.061 poss: Pyrrol compound: base mass 70, MW 154 **** 0.011 0.057unknown Pyrrol compound **** 0.006 0.030 unknown Pyrrol compound ****0.052 0.263 unknown Pyrrol compound **** 0.011 0.058 unknown Pyrrolcompound **** 0.012 0.061 other unknown compounds 0.059 0.304 no MS peakfound 0.009 0.048 no lib-spectrum found: base mass 57, MW 126 0.0080.041 unknown overlapping compounds 0.012 0.062 unknown overlappingcompounds 0.004 0.022 overlapping compounds 0.005 0.028 no lib-spectrumfound: base mass 99, 132 MW ? 0.005 0.023 no lib-spectrum found: basemass 42, MW ? 0.009 0.047 no lib-spectrum found: base masses 42, 98 MW ?0.004 0.019 no lib-spectrum found: base masses 69, 112 MW 140 0.0030.014 * = from library search with high quality, no standard, RRF = 1 **= isomeric or homologous compound, prefixes = ?, RRF = 1 *** = bestlibrary search spectrum, quality 50-70%, RRF = 1 **** = best librarysearch spectrum, quality 40-50%, RRF = 1/no library spectrum found

FIG. 2 shows a gas chromatography spectrum for the oily phase of thesame rapeseed biomass pyrolysis product as analysed in FIG. 1. The watercontent of the oily phase was determined to be 28.13%. The results ofanalysis of the spectrum are set out in the following Tables 3 and 4:

TABLE 3 Summary of oily phase wet oil dry oil Substance group wt. % wt.% Acids 0.218 0.303 Nonaromatic Alcohols 0.000 0.000 NonaromaticAldehydes 0.000 0.000 Nonaromatic Ketones 0.243 0.338 Furans 0.116 0.161Pyrans 0.000 0.000 Sugars 0.000 0.000 Benzene 0.222 0.308 Catechols0.042 0.059 Aromatic Aldehydes 0.000 0.000 Aromatic Ketones 0.000 0.000Lignin derived Phenols 0.822 1.144 Guaiacols/Methoxy phenols 0.040 0.056Syringols/Dimethoxy phenols 0.350 0.487 Nitrogen containing compounds3.267 4.546 Homologous aliphatic chains 1.951 2.715 other unknowncompounds 0.788 1.096 FID peak (no MS peak) 0.096 0.133 Miscellaneous0.000 0.000 Total 8.156 11.348

TABLE 4 Detailed analysis of oily phase Compound wt. % wt. % Acids 0.2180.303 Acetic acid 0.218 0.303 Nonaromatic Alcohols 0.000 0.000Nonaromatic Aldehydes 0.000 0.000 Nonaromatic Ketones 0.243 0.338Butanone, 2- 0.046 0.064 poss: 5-Hexene-2-one * 0.025 0.0343-Pentene-2-one, 4-methyl- * 0.102 0.142 2-Cyclopenten-1-one,2,3-dimethyl- 0.071 0.099 Furans 0.116 0.161 Furfuryl alcohol, 2- 0.1160.161 Pyrans 0.000 0.000 Sugars 0.000 0.000 Benzene 0.222 0.308 Toluene0.122 0.169 Benzene, ethyl- 0.033 0.045 Styrene 0.040 0.056 Benzene,butyl- * 0.015 0.021 Benzene, hexyl- * 0.012 0.016 Catechols 0.042 0.059Hydroquinone; Benzene, 1,4-dihydroxy- 0.042 0.059 Aromatic Aldehydes0.000 0.000 Aromatic Ketones 0.000 0.000 Lignin derived Phenols 0.8221.144 Phenol 0.227 0.316 Cresol, o-; Phenol, 2-methyl- 0.040 0.056Cresol, p-; Phenol, 4-methyl- 0.230 0.320 Cresol, m-; Phenol, 3-methyl-0.090 0.125 Phenol, 2-ethyl- 0.017 0.023 Phenol, 2,4-dimethyl- 0.0640.089 Phenol, 4-ethyl- 0.049 0.069 Phenol, ethyl-methyl- 0.013 0.019Phenol, 4-vinyl- 0.092 0.128 Guaiacols/Methoxy phenols 0.040 0.056Guaiacol, Phenol, 2-methoxy- 0.040 0.056 Syringols/Dimethoxy phenols0.350 0.487 Syringol; Phenol, 2,6-dimethoxy- 0.076 0.106 Syringol,4-ethyl- 0.171 0.238 Syringol, 4-vinyl- 0.102 0.143 Miscellaneous 0.0000.000 Nitrogen containing compounds 3.267 4.546 Pyridine * 0.022 0.031Pyridine, 2-methyl- * 0.015 0.021 Pyridine, dimethyl- MW 107 * 0.0070.010 poss: Butanenitrile, 3-methyl- *** 0.017 0.024 unknown aminecompound MW 99 **** 0.043 0.060 Pyrrole * 0.108 0.150 poss: Pyrazolecompound MW 112 **** 0.011 0.015 Pentanenitrile, 4-methyl- * 0.029 0.041similar to Pentanone, amino-methyl- (43, 58, 100) *** 0.902 1.255 poss:Pyrrole compound MW 137 **** 0.123 0.171 poss: Butanamide, 3-methyl- ***0.016 0.022 poss: unknown Nitroso-phenyl compound MW 148 **** 0.0200.028 poss: Piperidone, tetramethyl- MW 155 * 0.237 0.330 poss: unknownNitroso-phenyl compound MW 162 **** 0.020 0.028 poss: Benzyl nitrile MW117 *** 0.039 0.055 poss: 3-Pyridinol MW 95 *** 0.100 0.139Benzenepropanenitrile MW 131 * 0.024 0.033 Indole MW 117 * 0.122 0.170Indole, 3-methyl- MW 131 * 0.047 0.066 poss: 2,4-Imidazolidinedione,5-methyl- MW 114 *** 0.047 0.066 poss: Phenylisocyanate, dimethy- ****0.021 0.029 poss: unknown aliphatic Nitrile compound **** 0.015 0.021poss: Hydantoin or imidazolidinedione compound: base mass 100 MW ? ****0.066 0.092 poss: Hydantoin or imidazolidinedione compound: base mass100 MW ? **** 0.059 0.082 poss: Hydantoin or imidazolidinedionecompound: base mass 100 MW ? **** 0.017 0.024 poss: Pyrrol compound MW154 **** 0.033 0.045 poss: Hydantoin or imidazolidinedione compound:base mass 100 MW ? **** 0.016 0.023 poss: Pyrrol compound: base mass 70,MW 154 **** 0.284 0.395 poss: unknown aliphatic Nitrile compound ****0.031 0.043 unknown Pyrrol compound **** 0.035 0.049 aliphatic amidechain MW 212 *** 0.078 0.109 aliphatic amide chain MW ? *** 0.598 0.832aliphatic amide compound MW 238 *** 0.065 0.090 Homologous aliphaticchains 1.951 2.715 unknown saturated aliphatic chain ** 0.025 0.035unknown saturated aliphatic chain ** 0.016 0.022 unknown unsaturatedaliphatic chain MW ? ** 0.022 0.030 unknown saturated aliphatic chain **0.014 0.020 unknown unsaturated aliphatic chain MW ? ** 0.023 0.032unknown saturated aliphatic chain ** 0.021 0.029 unknown unsaturatedaliphatic chain MW ? ** 0.023 0.032 unknown unsaturated aliphatic chainMW ? ** 0.023 0.031 unknown unsaturated aliphatic chain MW ? ** 0.0270.037 unknown aliphatic chain ** 0.012 0.016 unknown unsaturatedaliphatic chain MW ? ** 0.017 0.023 unknown unsaturated aliphatic chainMW ? ** 0.032 0.045 unknown aliphatic chain ** 0.018 0.025 unknownsaturated aliphatic chain ** 0.014 0.020 unknown unsaturated aliphaticchain ** 0.024 0.033 unknown saturated aliphatic chain ** 0.033 0.046unknown unsaturated aliphatic chain ** 0.016 0.022 unknown unsaturatedaliphatic chain ** 0.035 0.049 unknown aliphatic chain ** 0.041 0.058unknown unsaturated aliphatic chain ** 0.054 0.075 unknown saturatedaliphatic chain ** 0.019 0.026 unknown unsaturated aliphatic chain **0.025 0.035 poss: Heptadecene MW 238 * 0.118 0.165 Heptadecene isomereMW 238 * 0.094 0.130 unknown saturated aliphatic chain ** 0.026 0.036poss: unknown aliphatic acid, methyl ester MW ? ** 0.015 0.022Octadecenoic acid, methyl ester MW 222 * 0.075 0.104 unknown aliphaticchain ** 0.032 0.045 poss: Octadecenoic acid (Oleic acid) ** 0.767 1.067unknown aliphatic chain ** 0.033 0.045 unknown aliphatic chain ** 0.0790.109 unknown aliphatic chain ** 0.181 0.251 other unknown compounds0.788 1.096 unknown compound: base mass 225, MW 240 **** 0.016 0.022 nolibrary spectrum found: base mass 58 **** 0.625 0.869 no libraryspectrum found: base mass 42, 95 MW ? **** 0.097 0.135 no libraryspectrum found: base mass 93, MW 186 **** 0.035 0.049 no libraryspectrum found base mass 58 . . . 128, 200 **** 0.015 0.020 FID peakwithout MS peak 0.096 0.133 no MS peak found 0.096 0.133 * = fromlibrary search with high quality, no standard, RRF = 1 ** = homologousaliphatic compounds, MS = ?, RRF = 1 *** = best library search spectrum,quality 50-70%, RRF = 1 **** = best library search spectrum, quality40-50%, RRF = 1/no library spectrum found

These results show that the vapour phase products of the pyrolysis arerich in lignin pyrolyse product. These compounds are not considered tobe particularly useful. Accordingly, it would be of benefit to reducethe fraction of these compounds and to increase the fraction of lowermolecular weight organic compounds. In effect, it is particularlypreferred to shift the peaks shown to the right of FIG. 2 further to theleft—this corresponds with lower molecular weight organic compounds.

The present inventors consider that one particularly suitable way toachieve this goal is to provide a pyrolysis process in which a largeproportion of char is provided in relation to the biomass. It isparticularly preferred to use a ratio of 1:3 biomass to char by weightin a continuous intermediate pyrolysis process. The char operates toprovide useful sites for cracking reactions during the pyrolysisprocess, so that at least some of the higher organics molecules arebroken down to lower molecular weight compounds. In addition, the charprovides the opportunity for the formation of syngas using the largequantity of water vapour present in the biomass reactor, by thereaction:C+H₂O→CO+H₂

FIG. 3 shows a schematic longitudinal cross sectional view of apyrolysis apparatus 10 according to a preferred embodiment of theinvention. The apparatus 10 includes a screw kiln having a first screwor auger 12 mounted on a rotatable shaft 14, the rotatable shaft 14 andfirst screw 12 rotating with respect to an inner cylindrical wall 16.Rotation of the first screw provides a means for conveying solidsaxially along the bore of the inner cylindrical wall 16. The innercylindrical wall 16 has a second screw 18 fixed to its outer surface.Inner cylindrical wall 16 is also mounted for rotation in the contrarydirection to the rotation of the rotatable shaft 14. Consequently,second screw 18 rotates with respect to an outer, fixed, cylindricalwall 20. Thus, rotation of the second screw provides a means forconveying solids axially along the space between the inner cylindricalwall 16 and the outer cylindrical wall 20.

In use, biomass feedstock is added into a feed inlet 22 which directsthe biomass feedstock to one end of the first screw 12. The biomassfeedstock is prevented from entering the space between the innercylindrical wall 16 and the outer cylindrical wall 20.

At the start-up of the process, pre-formed char may be added to thebiomass feedstock at the inlet 22 in order to achieve a preferred ratioby weight of biomass to char of 1:3.

The pyrolysis of the biomass takes place in the interior of thecylindrical space bounded by inner cylindrical wall 16. The biomass andchar is mixed and conveyed along the cylindrical space by the firstscrew 12. At the end of the cylindrical space distal from the inlet 22,there is a vapour outlet 24 and an ash outlet 26. The vapour outletconveys the vapour and gas phase products of the pyrolysis process.Preferably at least a portion of these products are conveyed to agasifier apparatus (not shown) for further processing.

The pyrolysis process also produces char, in addition to the char thatwas present prior to the pyrolysis process. At least a portion of thechar conveyed along the cylindrical space is allowed to fall via anaperture (not shown) into the annular space bounded by the innercylindrical wall 16 and the outer cylindrical wall 20. The action of thesecond screw 18 conveys this char in the reverse direction along theannular space. Inlets 28 for air and water may be provided along thisannular space. The purpose of these inlets is to introduce controlledamounts of air and/or water into the char in the annular space. Theoxygen in the air allows a portion of the char to combust, producing CO₂and heat. This heat drives the pyrolysis process (which is endothermic)in the cylindrical space. The introduction of water allows a reformingreaction to occur, producing syngas which can be extracted and used as afuel for electricity generation. The water may be steam or vapour, e.g.from drying of the biomass.

The remainder of the char is conveyed back towards the inlet end of theapparatus. Using a paddle (not shown) or similar device, the char thatreaches region 30 of the apparatus is lifted into the inlet end of thecylindrical space, in order to be conveyed along using the firstconveyor.

In this way, the apparatus uses the char produced in pyrolysis as a heatcarrier for subsequent pyrolysis, the high proportion of char to biomasspromoting the formation of a greater proportion of useful gaseousproducts for power generation.

Waste heat from the apparatus may be used to dry biomass beforeinserting it into feed inlet 22.

In the case where the apparatus is heated with a heat transfer medium ata high temperature (e.g. gasifier gases at temperatures of about 800° C.or combustion gas products at temperatures of about 1000° C.), such aheat transfer medium may be applied to the external surface of outercylindrical wall 20. Then, the char being conveyed by the secondconveying means may act as a heat transfer buffer. The char then buffersthe temperature “seen” by the pyrolysis reactor and by the fresh biomassbeing introduced into the pyrolysis reactor.

Although FIG. 3 shows the use of screw conveyor 12 to mix and convey thesolid material in the central cylindrical space, the present inventorsenvisage alternative embodiments, such as an embodiment using one ormore mixing ploughs to mix the char and the biomass feedstock, thereactor being heated by an external heat source (this embodiment is notillustrated). The ploughs operate to mix the solid content of thepyrolysis reactor, but do not provide a net conveyance of the solidcontent of the reactor, unlike the screw conveyor 12 of the embodimentillustrated in FIG. 3. When the system works in a continuous manner (asis preferred), the continuous addition of new biomass leads to thecontinuous formation of new char. An overflow for excess char isprovided, in order that the ratio of biomass : char by weight can bemaintained close to the ratio of 1:3. An outlet for the vapour and gasproducts is provided.

FIG. 4 shows a longitudinal schematic view of a further preferredembodiment of a pyrolysis apparatus according to the present invention.FIGS. 5-7 show longitudinal schematic views of the main components ofthe apparatus shown in FIG. 4, and so the features of these drawingswill be described together.

The pyrolysis apparatus 50 of FIG. 4 includes an outer cylindrical wall52 (see also FIG. 1). The apparatus is supported on supports 54, 56, 58and has electric heating means, such as resistance elements 60, 62formed around the outer cylindrical wall 52. In alternative embodiments(e.g. larger scale embodiments relying solely on combustion in order toprovide the heat required to drive pyrolysis), there may instead beprovided insulation around the outer cylindrical wall 52.

Towards a proximal end of the apparatus is provided a biomass inlet port64. Towards a distal, opposite, end of the apparatus is provided a gasand vapour products outlet port 66. Also towards this distal end of theapparatus is provided a char outlet port 68.

Within the space enclosed by the outer cylindrical wall is provided afirst screw auger 70 disposed within a second screw auger 80. Thesecomponents are shown more clearly in FIGS. 7 and 6, respectively.

In FIG. 6, the second screw auger has a substantially tubular wall 82with two helical screws 84, 86 extending along the outer surface of thetubular wall 82, from the proximal end to the distal end. These helicalscrews are adapted to scrape along the inner surface 53 of the outercylindrical wall 52 of the apparatus, when the second screw auger isrotated within the apparatus.

At the proximal end of the second screw auger are provided feedstockinlet slots 88. In this embodiment, four feedstock inlet slots areprovided, equispaced circumferentially and communicating through thetubular wall 82. Fewer or more inlets slots may be provided, as desired.

At the distal end of the second screw auger are provided outlet slots90. These slots allow fluid and solid material to escape from theinterior of the second screw auger. Corresponding slots 92 are providedin the helical screws, in order to avoid preventing solid material fromescaping freely from the interior of the second screw auger.

At the distal extremity of the second screw auger is provided a bearingsurface 94 for allowing rotation of the second screw auger in theapparatus. A corresponding bearing surface 98 is provided in thehousing. At the proximal extremity of the second screw auger is providedengagement means (e.g. screw holes 100) to allow rotation of the secondscrew auger to be driven by a corresponding driving means (e.g. driveplate 104 and drive shaft 106). In FIG. 5, drive shaft 106 is coaxialwith drive shaft 108, which is for driving the rotation of the firstscrew auger 70, described next.

First screw auger 70 includes a solid shaft 72 having a single helicalscrew 74 extending therealong. In use, the single screw engages withinner surface 83 of the tubular wall 82 of the second screw auger, inorder to convey material along the interior of the second screw auger.

At the distal end of the first screw auger is provided a bearing surface76, for rotational cooperation with a corresponding bearing surface 77of the housing. At the proximal end of the first screw auger is providedengagement means 78 to allow rotation of the first screw auger to bedriven by the corresponding drive shaft 108.

The housing includes a proximal end plate 110 and distal end plate 112.Proximal end plate 110 includes seal means 114 to provide a suitableseal between the proximal end plate 110 and the drive shaft 106. Furtherseal means 116 are provided to provide a suitable seal between thecoaxial drive shafts 106 and 108.

In use, biomass feedstock (e.g. wood chips) and a heat carrier (e.g.metal balls, ceramic balls or SiC balls) are delivered to inlet 64. Thesecond screw auger is rotated By rotation of drive shaft 106) in orderto provide a conveying direction from the distal end to the proximalend. Thus, the biomass feedstock are not conveyed by the second screwauger. Instead, the biomass feedstock and heat carrier gradually fallsthough the slots 88 in the proximal end of the second screw auger.

The first screw auger is rotated (by rotation of drive shaft 108) inorder to provide a conveying direction from the proximal end to thedistal end. Thus, the biomass feedstock and the heat carrier areconveyed and mixed along the interior of the second screw auger. Thispart of the apparatus is maintained at a temperature in the range300-600° C. in order to subject the biomass feedstock to pyrolysis.Typically, this process produces char (solid) and gaseous products andvapour products. The products are able to escape from the interior ofthe second screw auger via slots 90. The vapour and gaseous products areextracted from the apparatus via outlet 66. Some of the char and theheat carrier fall through slots 90 and are conveyed back along theapparatus (i.e. from the distal and to the proximal end) by the secondauger. The remainder of the char is conveyed to outlet 68. This char maybe recycled and used as heat carrier, or may alternatively be used incombustion to provide heat for the pyrolysis, or may still further beused for combustion in a CHP process.

In alternative embodiments, the heat carrier (typically balls) my besubstantially prevented from falling though slots 90 and may instead beconveyed to outlet 68. In this case, some or all of the heat carrierballs may be recycled for use again at the inlet 64 of the apparatus.

It should be noted that in other embodiments, the double helical systemdescribed with respect to FIGS. 4-7 need not be used. For example, twoseparate (but preferably adjacent) helical screws may be used as thefirst and second conveying means.

The apparatus 50 may be connected to an inlet of a gasifier apparatus.In particular, it may be advantageous to connect the outlet 66 of theapparatus to an inlet of a gasifier apparatus. For example, the gasifiermay itself consume biomass feedstock such as wood chips. The outlet 66from the pyrolysis apparatus may be provided to the pyrolysis zone ofthe gasifier. This provides benefits to the gasification process. Inparticular, it provides substantially ash-free pyrolysis vapours (andwater) into the correct location in the gasifier to improve theefficiency of the gasifier. It is considered that this can potentiallydouble the throughput for the same size downdraft gasifier.

Alternatively, the pyrolysis apparatus may be integrated with thegasifier apparatus. In this way, heat generated by the gasificationprocess may provide the heat necessary to drive the pyrolysis in thepyrolysis apparatus.

The pyrolysis apparatus may operate at an internal pressure aboveatmospheric pressure. For example, the pyrolysis apparatus may operateat 200-300 mbar over atmospheric pressure. This allows the vapourpyrolysis products effectively to be pumped into the gasifier apparatus,driven by the over pressure.

The pyrolysis apparatus may be pressurised from, e.g. 3 mbar for someapplications, or up to about 30 bar for other applications. This allowscoupling to a gasifier with its pressure drop. In place of the gasifier,or additional to the gasifier, the pyrolysis apparatus may be coupled toother pressurised reactors, such as a reformer or a hydrogenationsystem.

FIG. 8 shows a schematic view of a pyrolysis apparatus 50 integratedwith a gasifier apparatus 200. The gasifier apparatus 200 includes aninlet 202 for biomass, typically wood-based biomass. The biomass isgravity-fed into a pyrolysis zone 204 of the gasifier, there being anair inlet 206 slightly below the pyrolysis zone 204. Below the air inletin the gasifier is a reforming and gasification zone 208, above a charbed 210. At the base of the gasifier apparatus is ash section 212.

Outlet 66 of the pyrolysis apparatus 50 transfers pyrolysis vapour fromthe pyrolysis apparatus into the gasification zone 208, via duct 214.The gasifier has an outlet 216 for syngas generated by the gasifier.

The preferred embodiments have been described by way of example only.Modifications of these embodiments, further embodiments andmodifications thereof will be apparent to the skilled person and as suchare within the scope of the present invention.

The invention claimed is:
 1. A biomass pyrolysis process in whichbiomass feedstock is mixed with a heat carrier, the heat carrier atleast partly comprising char, the ratio by weight of biomass to charbeing 1:1 to 1:20, wherein the process is carried out in a screw/augerkiln.
 2. A biomass pyrolysis process according to claim 1 wherein theprocess is operated at a temperature of 600° C. or lower.
 3. A biomasspyrolysis process according to claim 2 wherein at least a portion of thechar produced in the pyrolysis process is recycled back into thepyrolysis process.
 4. A biomass pyrolysis process according to claim 2wherein at least a portion of the products of the pyrolysis process isconveyed to a gasifier apparatus.
 5. A biomass pyrolysis processaccording to claim 1 wherein at least a portion of the char produced inthe pyrolysis process is recycled back into the pyrolysis process.
 6. Abiomass pyrolysis process according to claim 1 wherein at least aportion of the products of the pyrolysis process is conveyed to agasifier apparatus.
 7. A biomass pyrolysis process according to claim 6wherein the gasifier apparatus operates at a temperature of at least700° C.
 8. A biomass pyrolysis process according to claim 6 whereinfurther biomass is introduced into the gasifier.
 9. A biomass pyrolysisprocess according to claim 8 wherein the further biomass introduced intothe gasifier is low ash biomass.
 10. A biomass gasification processincluding a biomass pyrolysis process according to claim 1, whereinvapour and/or gaseous products of the biomass pyrolysis process areprovided to an inlet of a biomass gasification process.
 11. A biomassgasification process according to claim 10 wherein the pyrolysis processis carried out at a pressure over atmospheric pressure of at least 50mbar.